Abstract
Chromatin state maps were developed to elucidate sex differences in chromatin structure and their impact on sex-differential chromatin accessibility and sex-biased gene expression in mouse liver. Genes in active, inactive, and poised chromatin states exhibited differential responsiveness to ligand-activated nuclear receptors and distinct enrichments for functional gene categories. Sex-biased genes were clustered by chromatin environments and mapped to DNase-hypersensitive sites (DHS) classified by sex bias in chromatin accessibility and enhancer modifications. Results were integrated with genome-wide binding data for five transcription factors implicated in growth hormone-regulated, sex-biased liver gene expression, leading to the following findings. (i) Sex-biased DHS, but not sex-biased genes, are frequently characterized by sex-differential chromatin states, indicating distal regulation. (ii) Trimethylation of histone H3 at K27 (H3K27me3) is a major sex-biased repressive mark at highly female-biased but not at highly male-biased genes. (iii) FOXA factors are associated with sex-dependent chromatin opening at male-biased but not female-biased regulatory sites. (iv) Sex-biased STAT5 binding is enriched at sex-biased DHS marked as active enhancers and preferentially targets sex-biased genes with sex-differences in local chromatin marks. (v) The male-biased repressor BCL6 preferentially targets female-biased genes and regulatory sites in a sex-independent chromatin state. (vi) CUX2, a female-specific repressor of male-biased genes, also activates strongly female-biased genes, in association with loss of H3K27me3 marks. Chromatin states are thus a major determinant of sex-biased chromatin accessibility and gene expression, with FOXA pioneer factors proposed to confer sex-dependent chromatin opening and STAT5, but not BCL6, regulating sex-biased genes by binding to sites in a sex-biased chromatin state.